You must learn him.
You must know the reason why he is silent. You must trace his weakest spots. You must write to him. You must remind him that you are there. You must know how long it takes for him to give up. You must be there to hold him when he is about to.
You must love him because many have tried and failed. And he wants to know that he is worthy to be loved, that he is worthy to be kept.
And, this is how you keep him.
(Originally from This is How You Lose Her, by Junot Díaz. Pronouns were originally female. This speaks to my heart, regardless of what the gender of the pronoun is.)